Populations, recruitment and competition
This chapter discusses the methodology of assessing populations, surveys of faeces (spraints) and dens, use of radio-nuclides, and direct observations. Estimates of population densities are presented from several species. Sea otter numbers plunged during the 19th century, recovered in the 20th century, and crashed again in 1990s. Gene flow is limited and genetic diversity low. Reproduction is low is all species, with small litter sizes (one in the sea otter) and long periods of cub dependency. Breeding seasons of Eurasian otters are related to food supply; annual variation of numbers of cubs is correlated with prey availability. The North American river otter and sea otter have a delayed implantation, the Eurasian otter does not. Competition between otter species is discussed, as is food competition with other piscivores, including mink, water mongoose, crocodile, alligator, and also with human fisheries.
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